DESCRIPTION: The conceptual issue addressed in this proposal is how neurons in the lateral superior olive (LSO) obtain their selectivity for interaural intensity disparities (IIDs), and whether neurons with different IID selectivities are arranged in an orderly fashion within isofrequency contours of the LSO. The impetus for this proposal derives from the success that investigators have had in discovering how medial superior olivary (MSO) neurons, and their avian analogue in the nucleus laminaris, obtain their selectivity for a particular interaural time disparity and how those features are systematically represented with those nuclei. In contrast, the way in which LSO cells obtain their individual sensitivity for a particular IID and the degree to which an orderly arrangement of IID sensitivities might exist in the LSO are issues that have received little attention and are poorly understood. Only Reed and Blum have proposed a hypothesis that addresses both the issue of how LSO cells obtain their individual IID selectivity and what structural features could also impart an orderly arrangement of IID selectivity within isofrequency contours. Their hypothesis, however, has never been tested experimentally. Moreover, it relies only on the differences in thresholds of the inputs from the two ears, and thus it does not incorporate a number of other important features of the LSO, such as latency and time-intensity trading. The aims of this proposal are to test the Reed and Blum hypothesis and fill in the gaps in our knowledge about how LSO neurons obtain their particular IID selectivity. The first series of experiments has two goals: 1) To survey the LSO and obtain an overall view of the prevalence of neurons whose IID selectivities are predicted by threshold differences between the excitatory and inhibitory ears. 2) To distinguish between the hypothesis based on threshold differences, and alternative hypotheses which predict that IID selectivity is determined by differences in the relative strengths of the excitatory and inhibitory inputs, differences in arrival times from the two ears, or by some combination of threshold, strength and latency differences. These hypotheses will be evaluated by obtaining indices of input thresholds, strengths and latencies for each ear and then confirming their relative contributions for creating the neuron s IID selectivity through time-intensity trading experiments. A second series of experiments will reveal the degree to which IID selectivities are arranged in an orderly fashion within one isofrequency contour in the LSO. These experiments will exploit the greatly hypertrophied 60 kHz isofrequency contour of the mustache bat s LSO, yielding data from a large number of cells within one isofrequency contour.